Spanish football's governing body on Thursday joined three elite clubs in their lawsuit against a 2 billion euro private equity deal agreed by administrators of its top divisions, deepening domestic divisions within the sport.

The RFEF federation said that, in alliance with Real Madrid, Barcelona and Athletic Bilbao, it sought the "annulment of the agreement and of the operation itself", which it said contained unspecified "illegalities".

All but five of the 42 clubs in Spain's top two divisions signed up to the 1.994 billion euro ($2.24 billion) investment from CVC Capital Partners, the first of its kind in Europe, in December.

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Four clubs, including the top-flight trio, opted out and one abstained from the deal. Dubbed La-Liga Boost, it buys CVC an 8.2% stake in a new company that will get broadcasting revenues and sponsorship rights for half a century.

The RFEF said in a statement that, via the lawsuit, it aimed to protect its rights and those of clubs, primarily non-professional ones, and to prevent the deal from "determining the direction of Spanish football for the next 50 years".

Real Madrid, Barcelona and Athletic Bilbao described the deal as "an illegal transaction that causes irreparable damage to the entire Spanish football sector" when they jointly announced the suit in December.

RFEF said it "shares the mindset of the three clubs ... with only La-Liga, together with the CVC fund, benefiting from this agreement (which...) increases inequality."

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The RFEF, when contacted by Reuters , declined to give more details about the lawsuit or what "illegalities" it was contesting.

The La-Liga-CVC partnership commits clubs to allocating 70% of funds on infrastructure and modernisation projects, up to 15% on signing players and the rest to reducing debt.

LaLiga executive director Oscar Mayo told Reuters on Tuesday that the league was "confident in the legality of the deal", suggesting the clubs' legal actions were related to the European Super League breakaway project.

Barcelona and Real Madrid were also among the driving forces behind last year's failed attempt to launch a Super League, a project they remain to committed to.